Trump, in an interview on Fox News, appeared to cast doubt on his willingness to defend the country, calling the people of Montenegro "strong" and "aggressive," suggesting that its aggressiveness could draw the US into World War III due to the NATO's mutual security clause enshrined in Article 5 of the NATO treaty. It deems an attack on one member of NATO an attack on all countries in the alliance.

Article 5 -- which is defensive -- aims to deter potential adversaries from attacking NATO members. During the Cold War, the main concern was the Soviet Union, but in recent years, Russia's aggressive actions in Eastern Europe have been the focus of attention. Ukraine and Georgia, the two countries Russia has invaded in the past decade, are not NATO members.

"Our collective defense clause, Article 5, is unconditional and iron-clad. It means that an attack on one is an attack on all. President Trump has made clear that the US is fully committed to NATO and our Alliance is stronger than ever," a NATO official told CNN.

The southeastern European country is the newest member of NATO, having joined the alliance in 2017, the Alliance's first expansion since 2009 when Albania and Croatia joined.

As NATO is a treaty alliance, the addition of new members requires approval of the US Senate and Montenegro's membership was supported by a large bipartisan majority, which voted 97 to 2 to allow the country to join. Trump also signed off on the country's joining soon thereafter.

Montenegro's strategic location on the Adriatic coast ensured that the entire coastline is now made-up exclusively of NATO members.

Russia opposed Montenegro joining NATO

Russia was vehemently opposed to Montenegro's joining the alliance, issuing statements of opposition. Moscow maintains close relationships with Montenegro's neighbor Serbia and Moscow has sought to maintain its influence in the region.

Serbia and Montenegro were once part of the same country, Yugoslavia, and later in a union with Serbia until Montenegro's vote for independence in 2006.

Additionally the government of Montenegro accuses Moscow of orchestrating an attempt to topple the government and assassinate the Montenegrin Prime Minister as part of an effort to stop its NATO bid.

Russia has recently been accused of similarly attempting to sabotage another Balkan country's NATO bid, Macedonia, with Greece expelling Russian diplomats over the issue.

Senate Armed Services Chairman John McCain said Trump "is playing right into Putin's hands" with his comments on Montenegro.

McCain tweeted, "The people of #Montenegro boldly withstood pressure from #Putin's Russia to embrace democracy. The Senate voted 97-2 supporting its accession to #NATO. By attacking Montenegro & questioning our obligations under NATO, the President is playing right into Putin's hands."

He added, "#Putin will do anything to shatter the transatlantic alliance. In 2016, he nearly succeeded in overthrowing #Montenegro's democratically elected government & murdering its prime minister in order to prevent it from joining #NATO"

Despite having a small military, only about 1,500 troops, Montenegro has been an active participant in NATO missions.

It currently has 20 troops serving in Afghanistan as part of the NATO-led training mission there.

Article 5 has only been invoked once to aid the US

Article 5 has only been invoked once, in support of the United States after the 9/11 attacks. This led to NATO's largest-ever military operation, in Afghanistan, where hundreds of thousands of Europeans and Canadians stood shoulder-to-shoulder with US troops and more than 1,000 paid the ultimate price," the NATO official told CNN.

Montenegro spends approximately 1.58% of its GDP on defense, making it one of the NATO members that does not currently hit the NATO recommended target of 2% of GDP. However, the country plans to reach that target by 2024 in accordance with the pledges made be all NATO allies at the Wales Summit. Trump has repeatedly slammed NATO members for not spending adequately on defense.

It's not the first time Trump's interaction with Montenegro has drawn scrutiny, Trump was famously seen on camera apparently pushing the country's Prime Minister, Du-ko Markovi-, out of the way immediately prior to a photo op involving all the NATO leaders at the 2017 summit.

The Prime Minister appeared to downplay Trump's comments Wednesday while responding to questions from parliamentarians.

"(Trump) said that the Montenegrin people (are) brave and that he does not want US citizens to fight for others and for other NATO member states. He did not say this only on that occasion but he also said the same at the NATO Summit," Markovi- said.

Trump's comments on Fox News stand in contrast to a statement issued by the White House in April 2017 following Trump's signing of the instrument of ratification that allowed for Montenegro's joining of NATO

"President Trump congratulates the Montenegrin people for their resilience and their demonstrated commitment to NATO's democratic values," the White House press secretary said.